Electronic devices are increasingly being incorporated into stretchable and/or wearable products. Applications such as medical sensors, media players, personal computers, or the like are being integrated into materials such as shirts, watches, caps, or any other compliant products. Typically, electronics that are incorporated into a stretchable and/or wearable products include a thin silicon die that is connected to conductive traces on a stretchable substrate.
An exemplary stretchable device 100 is illustrated in FIG. 1A. Stretchable device 100 includes a semiconductor die 120 formed on a stretchable substrate 110. The stretchable substrate 110 has a low elastic modulus that allows the device to stretch. For example, a suitable material for the stretchable substrate 110 may be polydimethylsiloxane (PDMS). Electrical components, such as processors, sensors, memory devices, or the like, may be formed on the semiconductor die 120. The semiconductor die 120 may be attached to a surface of the stretchable substrate 110 or encapsulated within the stretchable substrate 120. Die contacts 142 are each electrically coupled to a contact pad 144 with a wire bond 147. The interconnect pads 144 may be coupled to the contact pads 146 by a interconnect line 149. Since the interconnect lines 149 are typically formed with a ridged material, such as copper or other conductive stack of materials, the interconnect lines 149 may be formed in a meandering pattern. The meandering pattern allows for the interconnect lines 149 to be stretched without fracturing, similar to a spring.
However, stretchable devices are susceptible to damage during cyclic stretching, such as fracture or plastic deformations of the wire bonds 147 between the die contacts 142 and the interconnect pads 144, or in the interconnect lines 149 proximate to where there is a transition between the semiconductor die 120 and stretchable substrate 110. Such fractures 155 are illustrated in FIG. 1B. FIG. 1B is an illustration of the stretchable device 100 in the stretched state (as indicated by the arrows F) during one stretching cycle of a cyclical stretching process. The root cause of the failure is the sharp transition between the high modulus of the semiconductor die 120 and the low modulus of the stretchable substrate 110. The cumulative strain damage during cyclic stretching causes fractures 155 of the interconnections.